Several patents and publications are cited in this description in order to more fully describe the state of the art to which this invention pertains. The entire disclosure of each of these patents and publications is incorporated by reference herein.
Because solar cells provide a sustainable energy resource, their use is rapidly expanding. Solar cells can typically be categorized into two types based on the light absorbing material used, i.e., bulk or wafer-based solar cells and thin film solar cells.
Monocrystalline silicon (c-Si), poly- or multi-crystalline silicon (poly-Si or mc-Si) and ribbon silicon are the materials used most commonly in forming the more traditional wafer-based solar cells. Solar cell modules derived from wafer-based solar cells often comprise a series of about 180 to about 240 μm thick self-supporting wafers (or cells) that are soldered together and electrically connected. Such a panel of solar cells, along with a layer of conductive paste and/or connecting wires deposited on its surface, may be referred to as a solar cell assembly. The solar cell assembly may be encapsulated by, sandwiched between, or laminated between polymeric encapsulants. The resulting structure may be further sandwiched between two protective outer layers (i.e., front sheet and back sheet) to form a weather resistant module. The protective outer layers may be formed of glass, metal sheets or films, or plastic sheets or films. In general, however, the outer layer that faces to the sunlight needs to be sufficiently transparent to allow photons reach the solar cells.
In the increasingly important alternative, thin film solar cells, the commonly used materials include amorphous silicon (a-Si), microcrystalline silicon (μc-Si), cadmium telluride (CdTe), copper indium selenide (CuInSe2 or CIS), copper indium/gallium diselenide (CuInxGa(1-x)Se2 or CIGS), light absorbing dyes, organic semiconductors, etc. By way of example, thin film solar cells are described in e.g., U.S. Pat. Nos. 5,507,881; 5,512,107; 5,948,176; 5,994,163; 6,040,521; 6,123,824; 6,137,048; 6,288,325; 6,258,620; 6,613,603; and 6,784,301; and U.S. Patent Publication Nos. 20070298590; 20070281090; 20070240759; 20070232057; 20070238285; 20070227578; 20070209699; 20070079866; 20080223436; and 20080271675.
Thin film solar cells with a typical thickness of less than 2 μm are produced by depositing the semiconductor materials onto a substrate, generally in multiple layers. Further, connecting wires, metal conductive coatings, and/or metal reflector films may be deposited over the surface of the thin film solar cells to constitute part of the thin film solar cell assembly. The substrate may be formed of glass or a flexible film and may also be referred to as superstrate in those modules in which it faces towards the sunlight. Similarly to the wafer-based solar cell modules, the thin film solar cell assemblies are further encapsulated by, laminated between, or sandwiched between, polymeric encapsulants, which are further laminated or sandwiched between protective outer layers. In certain embodiments, the thin film solar cell assembly may be only partially encapsulated by the encapsulant, which means that only the side of the thin film solar cell assembly that is opposite from the substrate (or superstrate) is laminated to a polymeric encapsulant and then a protective outer layer. In such a construction, the thin film solar cell assembly is sandwiched between the substrate (or superstrate) and the encapsulant.
Encapsulants fulfill several important functions in the solar cell module. For example, they encase and protect the solar cell materials, which may be brittle or otherwise susceptible to physical insults, such as abrasion. In addition, in some solar cell modules the encapsulant adheres the solar cells to the module's outer layers. The need for durable, transparent, easily processible encapsulants has led to the investigation of thermoplastic polymers, such as ethylene vinyl acetate (EVA), poly(vinyl butyral) (PVB), and the ionomers of ethylene acid copolymers. These materials have a long history of use as interlayers in safety glass laminates, and therefore the advantages of their use as encapsulants in solar cell modules are readily apparent. These advantages include, for example, one or more of good optical properties, suitable stability, durability, ease of processability, resistance to penetration, and favorable economic factors. Moreover, those who manufacture safety glass laminates are well-suited to develop the skills and equipment necessary to laminate solar cell modules that incorporate these familiar materials as encapsulants.
In addition, cross-linking reactions have been investigated as a means to further improve the stability, durability and penetration resistance of thermoplastic polymeric encapsulants. For example, Japanese Unexamined Patent Publication 2003-212967 describes a thermosetting resin which, before being thermally cured, is fluid and can be formed into a thin film.
Cross-linkable ethylene vinyl acetate (EVA) has also been widely used as an encapsulant material in solar cell modules due to its low cost, high clarity, low modulus, low initial viscosity, low equilibrium moisture level, and good heat resistance. The use of cross-linkable EVA as encapsulant materials is not without disadvantages, however. For example, the liberation of acetic acid by EVA can lead to corrosion and yellowing of the EVA encapsulant. Also, peroxides are often incorporated in the EVA encapsulant as a reagent of the cross-linking reaction. Thus, the shelf life of the EVA encapsulant may be reduced by the peroxides' decomposition. Further disadvantageously, peroxides decompose to evolve oxygen, which may cause optical flaws such as bubbles to form in the EVA encapsulant.
Finally, these EVA sheets must be produced at comparatively low extrusion temperatures to prevent premature cross-linking, that is, cross-linking prior to lamination to form the solar cell module. Premature cross-linking may render the EVA unprocessible so that the lamination of the solar cell modules cannot take place at typical temperatures. A prematurely cross-linked EVA will not flow to conform to the solar cells and other components of the solar cell module, nor will it adhere the solar cells to the outer layers of the module.
It is therefore apparent that a need exists for a cross-linkable thermoplastic polymeric encapsulant material that can be processed in the melt at typical extrusion temperatures and typical lamination temperatures.